Betting on one's wife in any context related to sex is extremely inappropriate and unethical. It could lead to a breakdown in the relationship. The wife may feel objectified and violated, which would likely result in a loss of trust and respect within the marriage.
You can find 'bts bet fanfic' on some popular fanfic websites like Archive of Our Own (AO3). It has a large collection of BTS - related fanfics, and you can easily search for the ones with the 'bet' theme using the search bar.
Book fairs are another option. There are often booths dedicated to different countries' literatures, and you may be able to find some great Japanese novels there. You can also interact with sellers and get recommendations.
The bet could be about who can complete more difficult ninja missions in a month. Naruto is always eager to prove himself as a great ninja, and Sakura is highly competitive too.
You can try Fanfiction.net. It has a large collection of 'Draco and Hermione' fanfictions, and you can easily search for ones related to their bet. There are different categories and filters you can use to narrow down your search.
You can start from the very beginning, like your earliest memories. Maybe it was your first day at school or a special family event when you were a child.
She could have to do all the laundry for a week. This is a common and family - friendly bet consequence. It's a simple task that can be a result of losing a bet.